Typically, GaN-based nitride semiconductors are applied in the application fields of optical devices for blue/green LED (Light Emitting Diode) and electronic devices that are high speed switching, high-power devices such as MESFET (Metal Semiconductor Field Effect Transistor), HEMT (High Electron Mobility Transistors), etc.
Such GaN-based nitride semiconductor light emitting devices are mainly grown on a sapphire substrate or a SiC substrate. Then, a polycrystalline thin film of AlyGa1-yN is grown as a buffer layer on the sapphire substrate or the SiC substrate at a low growth temperature. Thereafter, an undoped GaN layer, an n-GaN layer doped with silicon (Si) or a combination of both is grown on the buffer layer at a high temperature to form a n-GaN layer. Further, a p-GaN layer doped with magnesium (Mg) is formed on the top to thus fabricate a nitride semiconductor light emitting device. And, a light emitting layer (active layer of a single quantum well structure or multiple quantum well structure) is formed as a sandwich structure between the n-GaN layer and the p-GaN layer.
The p-GaN layer is formed by doping Mg atoms during crystal growth. The Mg atoms implanted as a doping source during crystal growth should be substituted with Ga positions to act as a p-GaN layer. On the other hand, they are combined with a hydrogen gas separated from the source and a carrier gas to form a Mg—H complex in a GaN crystalline layer and become a high resistance material about 10 MΩ.
Therefore, after the formation of a pn junction light emitting device, there is needed a subsequent activation process for substituting Mg atoms with Ga positions by breaking the Mg—H complex. However, the light emitting device has a drawback that the amount of the carrier contributing to light emission in the activation process is approximately 1017/cm3, which is much lower than a Mg atomic concentration of 1019/cm3, thereby making it difficult to form a resistive contact.
To overcome this, there is utilized a method of lowering a contact resistance by using very thin transparent resistive metals to increase the current injection efficiency. However, the thin transparent resistive metals used to decrease the contact resistance are generally 75 to 80% in light transmission and a light transmission above this value acts as a loss. Further, there are limits in improving light output in a crystal growth of a nitride semiconductor without improving the design of the light emitting device and the crystallinity of a light emission layer and a p-GaN layer in order to increase inner quantum efficiency.
The aforementioned light emission layer is formed in a single quantum well structure or a multiple quantum well structure comprising pairs of well layers and barrier layers. Here, the respective pair of well layers and barrier layers comprising the light emission layer are constructed in a lamination structure of InGaN/GaN or InGaN/InGaN or InGaN/AlGaN or InGaN/AlInGaN.
At this time, materials of the well layer and barrier layer are determined respectively depending on the InGaN well layer, generally, an wavelength band of a light is determined by the indium composition of the InGaN well layer, which is dependent upon a crystal growth temperature, a V/III ratio and a carrier gas. Typically, a light emitting diode formed of a multiple quantum well layer of InGaN/GaN or InGaN/InGaN lamination structures is used, that is to say, a light emitting diode of a multiple quantum well structure utilizing an indium composition and the band engineering concept is used in order to form a light emission layer with a high internal quantum efficiency.
In an embodiment utilizing the band engineering concept, the InGaN/GaN quantum well structure effectively binds the carrier dropped in the InGaN well layer by using a relatively large GaN barrier layer, however, has a drawback that it is hard to obtain the crystallinity of the GaN barrier layer due to a low growth temperature. And, in manufacturing a light emitting diode formed of a multiple quantum well layer of InGaN/GaN lamination structures, there is a drawback that, as the number of periods increases, the number of crystal defects such as pits caused by the crystallinity of the GaN barrier layer is increase, rather than the light efficiency increases in proportion to the number of periods. Finally, there is a drawback that the light emitting layer which contributes light emitting is limited. Moreover in a p-GaN growth, by the formation of pits, Mg dopants are diffused into the pits of the light emission layer, thereby resulting in the breakdown of the interface between a final GaN barrier layer and a p-GaN nitride semiconductor, and affecting the light efficiency and the stability.
Furthermore, the light emission layer of the InGaN/InGaN lamination structure utilizing an indium composition increases the crystal growth temperature while relatively lowering the indium composition of the InGaN barrier layer to less than 5%, thus enabling it to obtain the crystallinity. However, the light emission efficiency is reduced due to a weak binding force of the carrier dropped in the InGaN well layer. Nevertheless, a good reliability can be obtained because crystal defects such as the formation of pits can be relatively suppressed.
Besides, in the event the GaN or InGaN barrier layer is applied to a multiple quantum well structure, although an improvement is expected in terms of leakage current, but this improvement is not resulted from the improvement of its crystallinity, but caused from an increase in operating voltage due to the connection of its resistance components in series. And, these resistance components generate subsequent heat to thus affect the reliability of the device and have a considerable effect on the life of the device.
Consequently, based on this related art, there is needed a new growth technique which guarantees the indium composition of a well layer, the crystallinity of a barrier layer and the concept of band engineering in order to improve the internal quantum efficiency of a light emission layer.